Coherently phased arrays have been in use in radar systems for many years. In these radar systems the phase of two or more antennas are locked together and the ensemble forms a diffraction pattern that can be altered by changing the antenna spacing, antenna amplitude, and relative phase relationships Analogous optical systems have recently been formed by actively or passively locking the phases of two or more identical optical beams. By adjusting the relative phases of each beam, the diffraction pattern formed by the optical ensemble can be changed.
The ability to adjust the intensity pattern of light in the far field simply by changing the relative phase or intensity has tremendous utility. Two useful consequences of this phase-intensity relationship are far-field beam steering and synthetic aperture shaping. Beam steering using phase control gives the possibility of fast, non-mechanical beam deflection. Synthetic aperture devices can be used to dynamically control focal length, multiple beams from the same aperture, and aberration correction all of which arise from the ability to arbitrarily control far-field intensity patterns using spatially registered phase control.
Phase control of coherent arrays is the most common method of far-field intensity shaping, but polarization control also plays a role in the intensity distribution. Standard coherently combined arrays employ beams which are all polarized along the same axis. In this case the polarization is simply a necessary condition for achieving a phased array. However if one allows the polarization to become a parameter, other possibilities arise.
One of the possibilities of polarization control in addition to phase control is the ability to form what are referred to here as discrete cylindrical vector (DCV) beams. Cylindrical vector (CV) beams have characteristics that distinguish them from Gaussian beams. Among these characteristics is a nonuniform polarization state across the beam. Namely, the polarization of CV beams vary uniformly around the center of the beam and can be radially polarized, azimuthally polarized, or some combination of the two. Additionally, CV beams have a characteristic null in the center of the beam which is required by the cylindrical symmetry. Applications of CV beams range from mitigation of thermal effects in high power lasers and laser machining to particle acceleration interactions. They can even be used to generate longitudinal electric fields when tightly focused due to the cylindrical symmetry of the polarization, and although they are typically formed in free space laser cavities using some variation of an axicon, they can also be formed and guided in fibers.
Typical methods for creating CV beams fall into two categories. The first category of techniques uses an intracavity axicon in a laser resonator to generate a CV mode. The second category starts with a single beam and rotates the polarization of portions of the beam to create an inhomogeneously (typically radially or azimuthally) polarized beam. This second method is used by Biss et al. in U.S. Pat. No. 7,151,632. The second method is also used by Schuster in U.S. Pat. No. 6,392,800 to create a minimally perturbative transformation from a single uniformly polarized beam to a radially polarized beam for microlithographic projection.
A method performed in accordance with the principles of the present invention introduces a third possibility for creating CV beams. The said method uses multiple beams to produce a composite beam which approximates a CV beam. A great utility offered by this novel method is that it can be used to generate discrete CV beams synthetically by, among other things, superimposing an array of distinct beams having successively varying polarization, as is described in detail infra. The pattern of beams is an approximation to a CV beam. Some of the advantages of a synthetically created CV beam are similar to the advantages of synthetic aperture radar over earlier radar techniques. For example, a synthetically generated CV beam created using a method in accordance with the principles of the present invention (a DCV beam) exhibits many of the features as any other type of phased array, such as dynamically controlled beam steering, aberration correction, etc.